A lithium-ion battery (sometimes LIB) is a member of rechargeable (secondary) battery types in which lithium ions move from a negative electrode to a positive electrode during discharge and back when charging.
LIBs have become integral to modern day portable electronic devices such as laptop computers and cellular phones. Moreover, with the advent of electric vehicles (EV) and plug-in hybrid electric vehicles (PHEV), there is a great demand for developing high energy density LIBs capable of long cruising distance of EVs and PHEVs.
Three kinds of carbons: graphite, soft carbon and hard carbon have been used for commercial LIBs as a negative electrode active material. Graphite may intercalate up to a maximum of one lithium atom per six carbon atoms under ambient conditions. Many of soft carbons heat-treated around 1200° C. show a maximum reversible capacity of about 300 mAh/g. Some hard carbons can intercalate up to over one lithium atom per six carbon atoms. From earlier reports of 400 mAh/g of reversible capacity, improvement of reversible capacity without an increase of irreversible capacity has been attempted and over 500 mAh/g of reversible capacity with a small irreversible capacity of about 60 mAh/g has been achieved.
Graphite is a three-dimensional ordered crystal. Soft carbon and hard carbon are constructed with two-dimensional ordered graphene sheets which are randomly stacked, that are called as a ‘turbostratic’ structure. Soft carbon is called as a graphitizing carbon because it can be relatively easily graphitized by heat treatment over 2000° C. On the other hand, hard carbon is hardly to be graphitized, even at 3000° C. under ambient pressure, so it is called as a hardly-graphitizable or non-graphitizing carbon. The raw material usually determines whether such a carbon is obtained under soft or hard condition.
Typical raw materials for soft carbon include petroleum pitch and coal tar pitch. Acenaphthylene can be used in the laboratory as a substitute for pitch. Hard carbon is obtainable by heat treating thermosetting resins such as phenolic resin, and vegetable fibers such as coconut shell. Some carbon materials heat treated at about 800° C. or less have a large capacity, but their discharging potential is too high to be used in current cells that the cell voltage will be lower than 3 V. The lithium-doping mechanism of these carbon materials is different from the mechanism under consideration here.
Furthermore, there is an increased demand for a high capacity as well as high lithiation rate capability with regard to negative electrode active (anode) materials for lithium ion batteries. In order to meet the demand, attempts were made to use metals or elements, such as Si and Sn, which can make an alloy with lithium as the anode material. Such metals or elements have a higher theoretical charge and discharge capacity than carbonaceous materials. However, such metals or elements have serious changes in volume, accompanied with charging/discharging of lithium and resulting in metal based anode active materials to creak and pulverize. Thus, when charging/discharging cycles are repeated, the metal based anode active materials show a sudden deterioration of capacity and a shorter cycle life.